Senso (to Become New Emperor) (践祚)
The term "Senso" means to succeed the position of Tenshi (emperor) upon the demise of the previous emperor or his abdication from the position. In ancient times, this term was written in kanji script as '践阼,' while '践' means to take an official position and '阼' means the position of the Tensi emperor. Senso is followed by a relevant proclamation, which is called enthronement.
Summary
Before the reign of Kanmu, the term Senso had been used to mean enthronement. The fifty-first Emperor, Heijo had executed Senso prior to the enthronement, and then the coronation followed.
Subsequently, the term Senso was likely distinguished from that of enthronement and coronation, and for this reason Emperor Chukyo, who had become the emperor by Senso without an enthronement ceremony or coronation, was called Hantei (halfway emperor) and was not bestowed no daijo-tenno-go (name of the retired emperor) after his retirement nor any shigo (posthumous name) after his demise (his shigo was given 636 years after his death.)
A series of ceremonies related to Senso are called "Senso-no-gi (ceremonies to become a new emperor)" which are regarded as a part of the emperor's constitutional functions, being composed of the first ceremony named Kenji (ceremony for imperial succession) to be executed immediately after the demise of the previous emperor, and the second ceremony named Sokuigo Choken-no-gi (ceremony to have an audience with the emperor after enthronement) for the Sanken-no-cho (chiefs of three organizations of power, namely the prime minister, the chairmen of House of Representatives and the House of Councillors, and the Chief Justice,) and being followed by the next ceremony named Sokui-no-rei (ceremony of the enthronement) which is to be held after the end of ryoan (period to mourn for the passing of emperor.)
Imperial succession requires for a new emperor to inherit the three sacred imperial treasures from the previous emperor.
The three sacred imperial treasures are composed of Yata-no-Kagami mirror, Yasakani-no-Magatama (comma-shaped jewel) and Ama-no-Murakumo-no-Tsurugi sword, each of which is enshrined in the Kyuchu-sanden (three sacred shrines in the imperial palace) including Kashiko-dokoro where the Yata-no-Kagami mirror is stored, while all of which, both jewels and shrines, are succeeded by the new emperor and the above said Kenji ceremony is held as a ritual for succession of Yasakani-no-Magatama and Ama-no-Murakumo-no-Tsurugi sword. Since the imperial throne itself may be approved by the retention of the three sacred imperial treasures, the Nanbokucho-Seijunron (argument on legitimacy of either Northern or Southern Court) has a basis for its denial of orthodoxy of Hokucho (Northern court) which ascended to the throne without the three sacred imperial treasures.
According to the Imperial House Act, the following are ordained:
Article 4. When the emperor has demised, the crown prince shall immediately assume the throne.
Article 24. When imperial throne is succeeded, the Sokui-no-rei (ceremony of the enthronement) shall be held.
The term 'Sokui (enthronement)' specified herein has the same concept as the term 'Senso (to become a new emperor)' used for the emperors from Emperor Heijo to Emperor Showa. The term 'Sokui (enthronement)' applied to the consecuutive lineage of emperors up to Emperor Showa is understood as equivalent to 'Sokui-no-rei (ceremony of the enthronement)' specified in the current Imperial House Act.